The present invention relates to calibrating an analog filter of a communication device, and more particular, to processing apparatus and methods that utilize a digital filter to compensate a frequency response of the analog filter.
FIG. 1 is a block diagram of a conventional wireless receiver 100. The wireless receiver 100 comprises an antenna 110, a low noise amplifier (LNA) 120, a mixer (MIX) 130, a filter 140, a programmable gain amplifier (PGA) 150, an analog-to-digital converter (ADC) 160, and a local oscillator (LO) 170. In a wireless communication system, a radio frequency (RF) signal SRF received by the antenna 110 usually comprises a desired signal component SRD and an undesired signal component SRU. The spectrum of the RF signal SRF is illustrated in FIG. 2A. The desired signal component SRD contains information desired by the wireless receiver 100, and the undesired signal component SRU comprises signals transmitted at adjacent channels and some un-modulated blocker signals that both interfere with the desired signal component SRD. Unfortunately, the signal strength of the undesired signal component SRU is usually much greater than the signal strength of the desired signal component SRD. It is therefore important to reject the undesired signal component SRU to avoid saturating the ADC 160.
As illustrated in FIG. 1, the RF signal SRF is amplified by the LNA 120 and then down-converted to an intermediate frequency (IF) band by MIX 130 and LO 170. The IF signal SIF is then filtered by the filter 140 that passes frequencies within a certain frequency range at IF band by a high-Q factor. The spectrum of the filtered IF signal SIF′ generated from the filter 140 is schematically illustrated in FIG. 2B. Please note that the signal strength of the undesired signal component SIU of the filtered IF signal SIF′ is suppressed. Therefore, the probability of the ADC 160 saturation caused by the undesired signal component SIU can be greatly reduced, and the information contained in the desired signal component SID can be fully obtained without damage or loss.
In a conventional design, the above-mentioned high-Q filter is implemented using a high-order analog filter, e.g., a 3-order Butterworth filter. The high-Q and high-order analog filters, however, have some drawbacks while utilized in the communication devices. For example, using the high-Q and high-order analog filters will result in a longer settling time of step response in the wireless receiver system, that is, delaying the wireless receiver system. In addition, when resistance or capacitance of the high-Q and high-order analog filter deviates from the original designated value due to unpreventable fabrication process variation, the 3 dB frequency of the high-Q and high-order analog filter will also have a deviation from the original designated value (e.g. deviation from 100 KHz to 120 KHz). Hence, backup capacitances are necessary for calibrating the analog filter in the analog domain, and an extra larger area is required.